1. Field of the Invention
The present invention relates to a technique that forms raster lines while carrying out sub-scan to print an image, and more specifically to a technique of extending a printable area in which an image can be recorded.
2. Description of the Related Art
Typical examples of the printer that forms raster lines while carrying out sub-scan so as to print an image on a printing medium according to input image data include a line printer that forms raster lines without main scan, which moves a head forward and backward relative to the printing medium, and a serial scan printer and a drum scan printer that form raster lines with the main scan of the head. These printers, especially ink jet printers use a nozzle array having a plurality of nozzles arranged in a sub-scanning direction for each color, with a view to enhancing the printing speed. The recent trend increases the number of nozzles arranged in the sub-scanning direction and thereby the size of the nozzle array, In order to attain the high-speed printing.
One recording method applied for such printers to improve the picture quality is the technique called the `interlace method` disclosed in, for example, U.S. Pat. No. 4,198,642 and JAPANESE PATENT LAID-OPEN GAZETTE No. 53-2040. FIG. 43 shows an example of the interlace method. A variety of parameters used in the following description are explained first. In the example of FIG. 43, the number of nozzles N used for dot creation is equal to 3. A nozzle pitch k [dots] represents the ratio of an interval between the centers of adjoining nozzles on the print head to a dot pitch w of the resulting recorded image. In the example of FIG. 43, the nozzle pitch k is equal to 2. Since each raster line or main scanning line is recorded by one pass of the main scan in the example of FIG. 43, the number of repeated scans s is equal to 1. The number of repeated scans s represents the number of passes of the main scan that enable each raster line to be filled with dots. As described later, when the number of repeated scans s is equal to or greater than 2, each pass of the main scan records the dots in an intermittent manner in the main scanning direction. The symbol L in FIG. 43 represents the amount of sheet feeding in the sub-scan and corresponds to 3 raster lines in this example.
The circles including two digits represent the recorded positions of the dots. The left digit denotes the nozzle number, and the right digit denotes the order of recording (that is, which pass of the main scan records the dot).
In the interlace method shown in FIG. 43, the first pass of the main scan creates dots on the respective raster lines with the nozzles #2 and #3, whereas the nozzle #1 does not create any dots. After the sheet feeding of 3 raster lines, the second pass of the main scan forms raster lines with the nozzles #1 through #3. The subsequent procedure repeats the sheet feeding of 3 raster lines and formation of raster lines by the respective passes of the main scan, so as to complete an image. The nozzle #1 does not form a raster line in the first pass of the main scan, because no dots are created on an adjoining raster line located immediately below the raster line by the second or any subsequent pass of the main scan.
The interlace method forms raster lines in this intermittent manner in the sub-scanning direction to complete an image. One major advantage of the interlace method is that the variations in nozzle pitch and ink spout characteristics can be dispersed on the resulting recorded image. Even if there are variations in nozzle pitch and ink spout characteristics, this method relieves the effects of the variations and thereby improves the picture quality. The example of FIG. 43 regards the case in which the respective raster lines are formed by one pass of the main scan at a specific nozzle pitch. The interlace method is, however, applicable to a variety of settings. For example, the amount of sheet feeding may be varied arbitrarily according to the nozzle pitch, the number of nozzles, and the number of repeated scans.
The interlace method is an extremely effective dot recording technique to improve the picture quality. This method, however, inevitably causes a non-printable area, in which an image can not be recorded, on the lower end of a printing medium when the recording starts from the upper end of the printing medium. FIG. 44 shows the state of dot creation according to the interlace method by the sheet feeding of 7 raster lines with a head having seven nozzles arranged at a nozzle pitch corresponding to 4 raster lines. The symbols P1, P2, . . . in FIG. 44 denote the passes of the main scan, for example, the first pass of the main scan and the second pass of the main scan. The circles including numerals represent the positions of the nozzles in the sub-scanning direction on each pass of the main scan. The encircled numerals denote the nozzle numbers. As a matter of convenience, raster numbers RN are allocated to the respective raster lines. The interlace method is adopted in this example, where each raster line is formed by one pass of the main scan at the corresponding nozzle position.
FIG. 44 shows six passes of the main scan in the vicinity of the lower end of the printing medium. The nozzle #7 in the pass P6 of the main scan is located at the lower-end limit position of the nozzle according to the mechanism of sheet feeding. The sheet feeding mechanism is described with the drawing of FIG. 4.
The sheet feeding mechanism of the printer generally includes two pairs of rollers in a feeding section and a delivering section of the printing medium. In the example of FIG. 4, the rollers in the feeding section of the printing medium include a feeding roller 25a and a follower roller 25b, whereas the rollers in the delivering section of the printing medium include a delivering roller 27a and a star-wheel roller 27b. The accuracy of sheet feeding in the sub-scan is generally ensured by either one of the two pairs of rollers in the feeding section and in the delivering section. In the case where the rollers in the feeding section ensure the accuracy of sheet feeding, the limit of image recording with the sufficient accuracy of sub-scan is the position at which the lower end of the printing medium comes off the feeding roller 25a and the follower roller 25b. The distance between the lower end of the head and the lower end of the printing medium at this moment is determined according to the positions of the feeding roller 25a and the follower roller 25b and is equal to the distance `a` shown in FIG. 4. The nozzle #7 in the pass P6 of the main scan in FIG. 44 corresponds to the nozzle at such a limit position.
When the image is recorded by the fixed amount of sheet feeding corresponding to 7 raster lines in this state, there is dropout of a raster line RN=-10 as shown in FIG. 44. Adoption of the interlace method accordingly causes the image to be recorded only up to the limit of an area A shown in FIG. 44. According to the combination of sheet feeding amounts in the interlace method, the printable area may be reduced to the position of the nozzle #1 in the pass P6 of the main scan (that is, the area of RN.ltoreq.-17) in the worst case. When the head has a width `h` in the sub-scanning direction, there is a non-printable area corresponding to the distance `a+h` from the lower end of the printing medium as shown in FIG. 4. The non-printable area is further extended, because the possible errors in sheet feeding require some additional margin.
The non-printable area is negligible in the case of a relatively small-sized nozzle array, that is, when the width `h` of the head shown in FIG. 4 is relatively small. The recent trend that increases the size of the nozzle array, however, results in a significantly large non-printable area. The large non-printable area significantly damages the advantages of the printer that records the image of high picture quality at a high speed.
After the printing medium comes off the rollers in the feeding section that ensure the accuracy of sheet feeding in the sub-scan, it is possible to continue the sub-scan with the rollers in the delivering section that give only the lower accuracy of sheet feeding. One possible procedure reduces the non-printable area by forming raster lines while carrying out such sheet feeding with the lower accuracy. For example, the pass P7 of the main scan shown in FIG. 44 solves the problem of dropout of raster lines and extends the printable area of the image. In principle, this technique enables the image to be recorded to the lower end of the printing medium.
The dot recording with the lower accuracy of sheet feeding in the sub-scan naturally lowers the picture quality. FIGS. 47 and 48 show a deterioration of picture quality when the accuracy of sheet feeding is lowered. FIG. 45 shows dots recorded in a predetermined area in the case where the sufficient accuracy of sub-scan is ensured. For better understanding of illustration, the raster lines filled with dots are shown alternately by the solid line and the broken line. In the state of FIG. 45, the dots are arranged at fixed recording pitches both in the main scanning direction and in the sub-scanning direction. The dots generally have the size that enables partial overlap with the adjoining dots. The predetermined area is accordingly filled with dots as shown in FIG. 45.
FIG. 46 shows dots recorded in the same area in the case where the sufficient accuracy of sub-scan is not ensured. Even in this case, the sufficient accuracy of main scan is ensured, so that the dots are created at a fixed recording pitch in the main scanning direction. The error in sub-scan, however, varies the recording pitch in the sub-scanning direction. This causes a part having a higher density of dots in the sub-scanning direction, such as an area a1, and a part having a lower density of dots, such as an area a2. The variation in density of dots is visually recognized as unevenness of density that is not included in the original image data and undesirably lowers the picture quality. In some cases, there is even dropout of dots like an area a3 in FIG. 46. The human vision is extremely sensitive to such dropout. The occurrence of such dropout thus significantly damages the picture quality. The interlace method is generally adopted to improve the picture quality, and such deterioration of picture quality is not negligible.
An increase in number of passes of the main scan in the interlace recording process prevents the occurrence of dropout of dots and improves the picture quality of the resulting image. The increase in number of passes of the main scan, however, undesirably lowers the printing speed. The performances of the printer depend upon both the picture quality of the resulting image and the printing speed. There have been no conventional techniques that improve the picture quality in the extended printable area without lowering the printing performances. This problem also arises in the printers that create dots without the main scan of the head.